(A) Field of the Invention
The present invention relates to a mask at frequency domain and a method for preparing the same and an exposing system using the same, and more particularly, to a mask at frequency domain, which has a plurality of amplitude patterns and a plurality of phase patterns generated from a circuit layout by Fourier transform process, and a method for preparing the same and an exposing system using the same.
(B) Description of the Related Art
FIG. 1 and FIG. 2 illustrate an exposing system 10 according to the prior art. From top to bottom, the exposing system 10 comprises an off-axis illumination (OAI) light source 12, a first lens 14, a mask 20, a second lens 16, a shielding plate 22, a third lens 18, and a wafer 30. The mask 20 includes a circuit layout 62 having two patterns 64 and 66, which are similar to the patterns to be formed on the wafer 30. Particularly, the difference between the patterns to be formed on the wafer 30 and the patterns 64, 66 is their sizes, i.e., the size of the patterns 64, 66 is smaller than that of the patterns to be formed on the wafer. In other words, the mask 20 can be regarded as a mask in spatial domain.
As the critical dimensions of integrated circuits are shrinking, it becomes increasingly difficult for the lithographic process to form a smaller pattern on the wafer 30 since it is more difficult to form patterns with a further reduced size on the mask 20. In addition, the prior art uses the OAI light source 12 to increase the depth of focus (DOF) of the exposing system 10. But the OAI technique can optimize the DOF for only one of these patterns 64 and 66 of the circuit layout 62, while the DOF for the other patterns is not optimized.